Plasma processing systems have long been employed to process substrates (e.g., wafers) in plasma processing systems. In a typical plasma processing chamber, plasma is ignited and confined in a plasma confinement region, which is typically defined by the chamber upper and lower strictures, as well as by structures that annularly surround the plasma confinement region.
To facilitate the insertion and removal of substrates, as well as to facilitate the evacuation of exhaust gas from the plasma processing chamber, many chambers employ a set of movable confinement rings to annularly confine the plasma. The movable confinement rings can be lifted upward, for example, to facilitate substrate insertion and removal. Generally speaking, the spacing between adjacent rings of the movable confinement rings is dimensioned to permit exhaust gas to be evacuated through the spacing while presenting a barrier to plasma expansion (e.g., by making the spacing smaller than the plasma sheath). In this manner, it is possible to physically constrain the plasma while allowing exhaust gas removal to occur through the set of movable confinement rings.
To facilitate discussion, FIG. 1 shows a simplified diagram of a portion of a prior art capacitively-coupled plasma processing chamber 100. There is shown a lower electrode 102 for supporting a substrate (not shown) during processing. Lower electrode 102 is typically powered by an RF power source (not shown) to generate and sustain a plasma 104. For process control purposes, it is desirable to confine plasma 104 within a plasma confinement region defined generally by lower electrode 102, upper electrode 106 (which may be grounded or powered by the same or another RF power source), and annularly by a set of confinement rings 110 (including rings 110a-d). As mentioned, gaps between confinement rings 110 allow exhaust gas to be pumped from the chamber while keeping the plasma confined within the aforementioned plasma confinement region. Confinement rings 110 may be made of a suitable material, such as quartz.
In the example of FIG. 1, there is also shown an annular grounded electrode 112 surrounding lower electrode 102. Annular grounded electrode 112 may be unslotted or may be slotted to provide additional flow channels for evacuating exhaust gas from the chamber as shown in the example of FIG. 1. Generally speaking, annular grounded electrode 112 is formed of a conductive material such as aluminum, and is electrically isolated from lower electrode 102 by an insulator (not shown). Grounding of grounded electrode 112 is accomplished by coupling grounded electrode 112 to an RF ground, typically by hard-bolting grounded electrode 112 or connecting it via one or more straps to a conductive lower ground extension that is disposed below lower electrode 112.
To prevent the metallic material of annular grounded electrode 112 from being exposed to the corrosive plasma and possibly contaminating the plasma process, the surface of annular grounded electrode 112 may be covered with a suitable material, such as quartz. As in the case with the set of confinement rings 110, the slots in annular grounded electrode 112 (and the overlying layer of quartz) are dimensioned to permit exhaust gas evacuation while preventing plasma from expanding beyond the plasma confinement region. The use of both confinement rings 110 and annular grounded electrode 112 in plasma processing chambers is known and will not be elaborated further here.
Generally speaking, confinement rings 110 are electrically floating, i.e., having no direct coupling to DC ground or RF ground. Since confinement rings 110 tend to be some distant away from RF ground in the prior art, no appreciable RF current flows through the set of confinement rings.
Since confinement rings 110 are left electrically floating and no appreciable RF current flows through confinement rings 110, a low voltage “floating” sheath is developed at the surface of confinement rings 110 during plasma processing. Since the energy of ions accelerated from the plasma is governed by the sheath potential, a low sheath potential results in a low energy level of ion bombardment on the surfaces of the confinement rings. Consequently, film removal processes such as sputtering and ion-enhanced etching (such as those occurring during in-situ plasma clean processes) are relatively inefficient at the surface of the confinement rings. Furthermore, a higher quantity of deposition is left on the surface of the confinement rings alter processing due to the low ion bombardment energy. By comparison, other regions of the chamber experiencing higher ion bombardment energy will see a higher film removal rate during film removal processes and a lower level of film deposition during substrate processing.
The net result is that the confinement rings tend to accumulate residues at a higher rate (relative to chamber regions that experience a higher ion bombardment energy) during substrate processing, and these residues tend to be removed more slowly (relative to chamber regions that experience a higher ion bombardment energy) during plasma in-situ chamber clean processes. These factors necessitate more frequent and/or longer in-situ chamber clean cycles (such as waferless auto-clean or WAC cycles) to keep the confinement rings in a satisfactory condition and may even require, in some cases, halting processing altogether so that the confinement rings can be removed and cleaned and/or replaced. As a consequence, the substrate throughput rate is disadvantageously reduced, leading to lower productivity and a higher cost of ownership for the plasma processing tool.
In certain chambers, control of the exhaust gas conductance rate (i.e., the rate at which exhaust gas is evacuated from the chamber) is an important control knob for controlling the wafer area pressure (WAP), i.e., the pressure above the substrate during processing. A high exhaust gas conductance rate tends to result in a lower wafer area pressure and vice versa. Given that wafer area pressure is a critical process parameter to control in many processing applications, improved methods and apparatus for controlling the wafer area pressure are constantly sought after by process engineers.
Embodiments of the invention seek to address one or more of the residue problem associated with prior art plasma confinement mechanisms, the plasma unconfinement issue, and the need to more efficiently control the wafer area pressure via exhaust gas conductance rate control.